A flask for the transport of high level vitrified radioactive waste operates dry under all conditions. The waste is contained in sealed containers which are housed within the flask and it is necessary to provide for thermal transfer between the containers and the flask wall in order to dissipate the heat generated by radioactive decay of the waste. It is known to provide a monolithic support structure within the flask to receive the sealed containers. Heat transfer from the waste through the support structure and into the flask wall is determined by the degree of thermal contact between the components. Due to the size and complexity of a monolithic support structure, good thermal contact is often difficult to achieve and distortion of the structure during use can make subsequent withdrawal from the flask a difficult operation.